fix: current speed stuck at 0 B/s due to unstable spring animation (#30)

animate.Spring multiplied velocity by 1 - damping*dt. At the UI tick
interval (130ms) this evaluates to 1 - 12*0.13 = -0.56, a negative
damping factor that flips the velocity sign and grows its magnitude
every tick. The animated throughput value collapsed to deep negative
values, which FormatBpsExt clamps to zero — so the current download/
upload readout stayed at 0 B/s while the sparkline and peak (which read
raw history samples) showed real traffic.

Damping now uses exp(-damping*dt), which stays positive for any step
size, keeping the spring stable and convergent. Adds a regression test
at the UI tick interval.

Bump VERSION to 0.2.2.

Closes #29

Author: programmersd21

CHANGELOG.md

@@ -1,3 +1,11 @@
+## [0.2.2] - 2026-08-18
+
+### Fixed
+- Current download/upload speed always displaying `0 B/s` — the spring animation in `animate.Spring` was numerically unstable at the UI tick interval (130ms), since `1 - damping*dt` evaluated to a negative damping factor. Velocity flipped sign and grew every tick, driving the animated value deeply negative, where `FormatBpsExt` clamped it to `0 B/s`. Damping now uses an exponential factor (`exp(-damping*dt)`), keeping the spring stable and convergent at any step size. The sparkline and peak values were unaffected because they read raw history samples directly.
+
+### Changed
+- VERSION bumped to 0.2.2.
+
 ## [0.2.1] - 2026-07-23
 
 ### Added

VERSION

@@ -1 +1 @@
-0.2.1
+0.2.2

cmd/flow/main.go

@@ -16,7 +16,7 @@ import (
 	"github.com/programmersd21/flow/internal/ui"
 )
 
-var version = "0.2.1"
+var version = "0.2.2"
 
 func main() {
 	flagTiny := flag.Bool("tiny", false, "single-line mode for tmux/status bars")

internal/animate/ease.go

@@ -32,7 +32,7 @@ func Clamp01(t float64) float64 {
 func Spring(current, target float64, velocity *float64, dt float64) float64 {
 	force := stiffness * (target - current)
 	*velocity += force * dt
-	*velocity *= 1.0 - damping*dt
+	*velocity *= math.Exp(-damping * dt)
 	if *velocity < 0.0001 && *velocity > -0.0001 {
 		*velocity = 0
 	}

internal/animate/ease_test.go

@@ -48,3 +48,22 @@ func TestSpringConverges(t *testing.T) {
 		t.Errorf("Spring did not converge: %f (want ~100)", val)
 	}
 }
+
+func TestSpringStableAtUITickInterval(t *testing.T) {
+	const target = 34603008.0
+	var vel float64
+	val := 0.0
+	minVal := val
+	for i := 0; i < 1000; i++ {
+		val = Spring(val, target, &vel, 0.13)
+		if val < minVal {
+			minVal = val
+		}
+	}
+	if math.Abs(val-target) > target*0.01 {
+		t.Errorf("Spring did not converge at dt=0.13: %f (want ~%f)", val, target)
+	}
+	if minVal < 0 {
+		t.Errorf("Spring oscillated negative at dt=0.13 (min %f) — value would render as 0 B/s", minVal)
+	}
+}